With continual improvement of fabricating technology of photosensitive elements (CMOS/CCD), image capturing devices are being developed towards the trend of high resolution and small size. Common image capturing devices, such as phone cameras, Digital Video Cameras (DVC), Digital Still Cameras (DSC) and Digital Single Lens Reflex (DSLR) cameras, with increasing resolution are being developed to satisfy the needs of all kinds of customers.
As to the current technology trend of image capturing devices, in addition to decreasing of the unit size of sensitivity element and the increasing of sensitivity thereof, in order to meet the requirements of high-resolution still images and film development, increase of the number of pixels in an image is also desirable; and so as increase of the storage capacity of digital storing mediums and enhancement of compression for digital signals. It may also be desirable to provide dynamic images having enough resolution to satisfy the need to play high quality images. In other words, the current development trend is to provide light and compact products having high zoom capacity and luminance. It is therefore desirable to develop zoom lens assembly suitable for image capturing devices with medium/high resolution and medium/high photosensitive elements.
The standard features of a camera, such as size, diaphragm size (Fno), field of view (FOV), zoom ratio and numbers of lens, cannot be simultaneously optimized under the objective of miniaturization. For example, U.S. Pat. No. 5,285,316, No. 5,396,367, No. 5,978,152, No. 5,359,457, No. 5,712,733, No. 5,940,221 and No. 5,612,825 all employ a zooming design with four lens groups.
U.S. Pat. No. 5,285,316 discloses a design having zoom ratio of 7 (6.3 mm˜47.972 mm) and diaphragm size up to 2 (Fno˜2), wherein the focal length of the four lens groups are respectively arranged as positive, negative, positive and positive. The design employs ten pieces of lens to construct the four lens groups wherein the third lens group is stationary, and two aspheric surfaces are provided. Thus a design having image height of 3.3 mm, total length of 67 mm and diaphragm of 14.85 mm is provided. U.S. Pat. No. 5,396,367 discloses a design having zoom ratio of 10 (6.3 mm˜60.43 mm) and Fno˜1.85, wherein the focal length of the four lens groups are respectively arranged as positive, negative, positive and positive. The design employs ten pieces of lens to construct the four lens groups wherein the third lens group is stationary, and 3 aspheric surfaces are provided. Thus a design having image height of 3.3 mm, total length of 73 mm and diaphragm of 15.85 mm is provided. U.S. Pat. No. 5,978,152 discloses a design having a zoom ratio of 10 (6.3 mm˜61.244 mm) and Fno˜1.46, wherein the focal length of the four lens group are respectively arranged as positive, negative, positive and positive. The design employs ten pieces of lens to construct the four lens groups wherein the third lens group is stationary, and four aspheric surfaces are provided. Thus a design having image height of 3 mm, total length of 70.2 mm and diaphragm of 16.3 mm is thus provided. U.S. Pat. No. 5,359,457 discloses a design having a zoom ratio of 7 (6.3 mm˜47.57 mm) and Fno˜1.64, wherein the focal length of the four lens groups are respectively arranged as positive, negative, positive and positive. The design employs nine pieces of lens to construct the four lens groups wherein the third lens group is stationary, and two aspheric surfaces are provided. Thus a design having image height of 3 mm, total length of 82.2 mm and diaphragm of 22.02 mm is thus provided. U.S. Pat. No. 5,712,733 discloses a design having a zoo ratio of 12 (6.3 mm˜73 mm) and Fno˜1.75, wherein the focal length of the four lens group are respectively arranged as positive, negative, positive and positive. The design employs ten pieces of lens to construct the four lens groups wherein the third lens group is stationary, and 2 aspheric surfaces are provided. Thus a design having image height of 4.68 mm, total length of 108.4 mm and diaphragm 25.42 mm of diaphragm is provided. U.S. Pat. No. 5,940,221 discloses a design having a zoom ratio of 12 (6.3 mm˜84.25 mm) and Fno˜1.47, wherein the focal length of the four lens groups are respectively arranged as positive, negative, positive and positive. The design employs ten pieces of lens to construct the four lens groups wherein the third lens group is stationary, and four aspheric surfaces are provided. Thus a design having image height of 4.067 mm, total length of 96.4 mm and diaphragm of 25.79 mm is provided. U.S. Pat. No. 5,612,825 discloses a design having a zoom ratio of 12 (6.3 mm˜73.82 mm) and Fno˜1.8, wherein the focal length of the four lens groups are respectively arranged as positive, negative, positive and positive. The design employs twelve pieces of lens to construct the four lens groups wherein the third lens group is stationary, and one aspheric surface is provided. Thus a design having image height of 4 mm, total length of 85.8 mm and diaphragm of 20.247 mm is provided.
However, in the above conventional techniques, only the second and/or fourth lens groups are designed to be movable, which lack variability in zooming. In the cases of increasing resolution and luminance, the overall size of devices cannot be decreased, the zooming capacity is therefore limited such that image with appropriate zooming cannot be achieved. Furthermore, the above conventional techniques employ considerable number of lens, which increases the fabrication cost thereof.
Thereafter, U.S. Pat. No. 6,788,474, No. 6,587,281, No. 6,714,355, and No. 6,538,825 respectively disclose a design wherein the second, third, and fourth lens groups are movable.
U.S. Pat. No. 6,788,474 discloses a design having a zoom ratio of 10 (5.864 mm˜58.871 mm) and Fno˜2.8, wherein the focal length of the four lens groups are respectively arranged as positive, negative, positive and positive. The design employs eleven pieces of lens to construct the four lens groups wherein the third lens group is movable, and 2 aspheric surfaces are provided. Thus a design having image height of 3.71 mm, total length of 78.1 mm and diaphragm of 19 mm is provided. U.S. Pat. No. 6,587,281 discloses a design having a zoom ratio of 10 (5.864 mm˜58.871 mm) and Fno˜2.8, wherein the focal length of the four lens groups are respectively arranged as positive, negative, positive and positive. The design employs eleven pieces of lens to construct the four lens groups wherein the third lens group is movable, and 2 aspheric surfaces are provided. Thus a design having image height of 3.71 mm, total length of 78.1 mm and diaphragm of 19 mm is provided. U.S. Pat. No. 6,714,355 discloses a design having a zoom ratio of 3 (3.643 mm˜10.42 mm) and Fno˜2.8, wherein the focal length of the four lens groups are respectively arranged as positive, negative, positive and positive. The design employs ten pieces of lens to construct the four lens groups wherein the third lens group is movable, and 1 aspheric surface is provided. U.S. Pat. No. 6,538,825 discloses a design having a zoom ratio of 3 (8.054 mm˜23.74 mm) and Fno˜2.8, wherein the focal length of the four lens groups are respectively arranged as positive, negative, positive and positive. The design employs nine pieces of lens to construct the four lens groups wherein the third lens group is movable, and 2 aspheric surfaces are provided. Thus a design having image height of 4 mm is provided.
However, the zoom ratio in the case of the abovementioned designs with movable second, third and fourth lens groups is generally not enough, and the maximum diaphragm size (Fno) is too high or the number of lens is too many. The problem of miniaturizing the zoom lens assembly is still not solved in the abovementioned US patents. At the same time, the US patents cannot simultaneously increase the luminance and the FOV and/or decrease the number of lens while reducing the size and fabrication cost thereof. Additionally, zoom lens assembly fabricated for different product classes cannot be provided.
Consequently, there is a need for a technique that provides compact zoom lens assembly having high luminance and low fabrication cost to overcome the drawbacks of the prior art.